Strain gages are commonly used to detect stresses in materials, changes in pressure and temperature, etc. Typically, strain gages employ one or more piezoresistive elements or piezoresistors which experience a change in resistance when subjected to strain induced by physical and/or chemical stimuli. The piezoresistive elements found in a conventional strain gage are usually formed of several loops of fine wire or a special foil composition. In use, the gage is bonded to the surface of the object to be analyzed. When the object is deformed in response to particular stimuli, the piezoresistive elements of the gage are strained which alters the resistance of the piezoresistive elements. The change in resistance is measured and then is correlated to the level of strain experienced by the object.
Recently, micro-electromechanical sensors have been developed that are manufactured by semiconductor microelectronic processing and precision etching technologies. These sensors can be employed for measuring parameters such as pressure, acoustic vibrations, inertia (acceleration, vibration, shock), gas concentration, temperature etc. Such sensors typically employ micromechanical elements (membranes, cantilever beams, microbridges, tethered proof masses, etc.) which are perturbed by physical and/or chemical stimuli, with the magnitude of the perturbation being related to the magnitude of the physical or chemical stimuli. Typically, piezoresistors are positioned on the micromechanical element at high-stress locations of the micromechanical element (for example, at the edge of a membrane). The sensitivity of such sensors is proportional to the piezoresistive gage factor of the piezoresistors, defined as: GF=xcex94R/Rxcex5, the relative change in resistance xcex94R/R with strain xcex5.
Silicon is a common material for forming the piezoresistors in micro-electromechanical sensors and has a gage factor that is suitable for various applications. However, in some instances, a higher gage factor is desirable so that the sensitivity of the sensor incorporating the piezoresistor can be increased, or alternatively, the micromechanical element on which the piezoresistor is positioned, can be stiffened for increased mechanical strength without reducing the sensitivity of the sensor. Attempts have been made to produce piezoresistors with higher gage factors than silicon, however, such attempts have not produced significantly higher gage factors with consistant piezoresistive properties.
The present invention is directed to a piezoresistor having a gage factor that is significantly higher than current piezoresistive devices. The piezoresistor of the present invention includes a base substrate with a quantum well structure formed on the base substrate. The quantum well structure has at least one quantum well layer bound or sandwiched by barrier layers. The barrier layers are formed from a material having a larger bandgap than the at least one quantum well layer.
In preferred embodiments, each quantum well layer in the quantum well structure is less than about 1000 xc3x85 thick and is more preferably about 5 xc3x85 to 30 xc3x85 thick. The barrier layers in the quantum well structure are less than about 1000 xc3x85 thick and are more preferably about 5 xc3x85 to 50 xc3x85 thick. The quantum well structure may have one or more quantum well layers with about 5 to 10 layers being preferred. Selected layers in the quantum well structure may be doped. The base substrate is preferably a single crystal and the layers of the quantum well structure are formed by epitaxial growth.
In one embodiment, the base substrate is a wafer, thin film or thin foil. In another embodiment, the base substrate is a micromachined mechanical element of a sensor on which the quantum well structure is fabricated.
The present invention piezoresistor can be fabricated from semiconductor materials which are resistant to harsh conditions, such as, high temperatures, high pressure, and corrosive or reactive gas environments. Such semiconductor materials are typically compatible with microelectronic devices and can be manufactured in batches. This makes the present invention piezoresistor low cost in comparison to metal strain gages. In addition, the piezoresistor can be fabricated either as a strain gage that is bonded to an object or structure to be analyzed, or as part of an electromechanical sensor for measuring parameters, such as, pressure, acceleration, vibration etc.